Polyesteramides, which have amide functionality incorporated into a polyester in a uniform fashion are of current interest for several reasons. At high levels of amide loading (&gt;5%), the melting point and glass transition temperatures of the polymers can be significantly increased. See R. J. Gaymans and J. L. de Haan, Polymer, 34, 4360 (1993); (b) A. van Bennekom, "Fast Crystallizing Polyesteramides," Thesis for University of Twente (1995). Even at low levels of amide functionality (&lt;1%), enhanced crystallization rates and increases in heat distortion temperature are observed. Unfortunately, it has proven difficult to prepare the polyesteramide polymers, due to side reactions observed upon introduction of a diamine into a polyester synthesis. For example, attempts to combine dimethyl terephthalate with butanediamine in the melt lead to significant levels of methylation on the amine (as much as 25%), which precludes efficient high molecular weight polymer formation upon introduction of a diol.
Currently, polyester amide polymers are prepared by a two step process, in which an intermediate bis-esteramide such as butylene-1,4-bis(p-carbomethoxy benzoyl amide) (T4T) is first prepared and purified, followed by utilization of that bis-esteramide as a comonomer with DMT and butanediol in the polymerization step.
Bis-esteramides have been prepared by several processes. Conversion of monomethyl-terephthalate to its acid chloride, followed by reaction with a diamine leads to high yields of bisesteramide. See I. Goodman, Eur. Pol. J. 27, 515 (1991). This route is not commercially viable due to limited availability of the monoacid, and to the necessity of its conversion to acid chloride.
Amide ester exchange of diamines on dimethyl terephthalate has been extensively studied by the Gaymans group at the University of Twente. See R. J. Gaymans and J. L. de Haan, Polymer, 34, 4360 (1993); A. van Bennekom, "Fast Crystallizing Polyesteramides," Thesis for University of Twente (1995) and Hotten, R. W. Chem. Ind. Eng. 1957, 49, p1691. According to the Gaymans process, lithium methoxide is used as the base in an amide ester exchange with excess dimethyl terephthalate (DMT). The use of relatively large amounts of lithium salts requires undesirable purification to remove them.
A presently preferred methodology for preparation of the bis-esteramide involves reaction of a 5-10 moles of DMT with butanediamine in a mixture of dry methanol/toluene using lithium methoxide as a catalyst at levels of up to 30% molor, based on diamine. This process is disclosed in U.S. Pat. No. 5,510,451 to Gaymans, et al. According to the patent a bisester diamide is prepared in a first step by reaction of a diamine with at least two-fold molar quantity of a diester of terephthalic acid, for example, dimethyl terephthalate. This reaction is generally carried out in the presence of a catalyst, for example, Li(OCH.sub.3). A mixture of the diester diamide, a diol and optionally terephthalic acid, or a terephthalic acid derivative, may then be condensed to form a prepolymer. This prepolymer may finally be postcondensed to form a copolyester amide having the desired properties.
The above processes are, in general, complex and uneconomical, especially in view of the high amounts of catalyst utilized, sine the various intermediate products have to be isolated and purified. Hence, it is desirable to develop a simpler and more economical process which requires fewer process steps without complex purification of intermediate products.